The year is 1924 and Ramsey McDonald has just been elected as prime minister. With the Labor Party’s antipathy toward the cosseted likes of the Crawley family, Sir Robert (Hugh Bonneville) is having the upper-crust equivalent of a meltdown in Sunday night’s fifth-season premiere of the Julian Fellowes drama.

Wearing a zip-up sweatshirt and corduroys, a very jolly Bonneville sits down at the Lamb’s Club in midtown to discuss the momentous changes that await the Crawleys, as the world clock brings them closer to the Great Depression of 1929.

“There was a real concern that a socialist government would tax the gentry to extinction,” says Bonneville. “Or be as radical as what had happened in Russia four years previously, in terms of the expulsion of that strata of society. Robert is absolutely obsessed that the estate is about to be ravaged.”

The 51-year-old actor is visiting New York with select members of the “Downton” cast, attending screenings for select fans and politely answering their questions. The phenomenon of the show — the most popular in PBS history — has transformed his life.

He is now offered parts in movies starring George Clooney (“The Monuments Men”) and marketed by the Weinstein Company (the forthcoming family film “Paddington”).

And he is not even remotely tempted to tempt fate, even if that means asking to see advance scripts.

“I had a word with [executive producer] Gareth [Neame] yesterday that Julian had delivered the first two episodes for the new season and they’re in really good health,” he says. “I don’t even want to quiz. I probably should be analyzing the character arc and fighting for more or less. But I just love diving in and seeing what’s going to happen.”

What’s going to happen is that Sir Robert is going to clash with the nearest socialist in town, a firebrand schoolteacher named Sarah Bunting (Daisy Lewis), a friend of his son-in-law, Tom (Allen Leech). An invitation is arranged for her attend a rather important family event.

And they don’t waste much time getting into an argument about politics.

Daisy Lewis portrays Miss Sarah Bunting and Allen Leech is Tom Branson. “She’s just a nasty piece of work from Robert’s point of view,” says series star Hugh Bonneville of Lewis’ role.Nick Briggs/Carnival Film & Television Limited

“She’s just a nasty piece of work from Robert’s point of view,” Bonneville says. “She utterly despises the aristocracy, and cannot understand why Tom Branson would want to be friends with any of them. And she comes to his house and she’s bloody rude to him. It was great fun playing it because finally, finally, he snaps. I think the stage direction read, ‘He flings down his napkin and the glass breaks.’”

The intrusion of the modern world and the introduction of a more modern morality finds members of the Crawley family at the crossroads, particularly poor Lady Edith (Laura Carmichael), who gave up an out-of-wedlock daughter for adoption to a tenant farmer on the estate. Bonneville sees Edith’s reluctance to confide in her parents as typical of the times.

“It was the era in which you did not divorce if you could help it. A woman certainly wasn’t allowed to,” he says. “And homosexuality was illegal. To have a child out of wedlock: My goodness. It was social suicide, really. The level of shame that would fall on the family would be appalling. Even though we, the audience, know that Cora is a very accepting character. Eventually Robert will find out, I’m sure.”

Before the arrival of the grim day, Robert Crawley is happy to watch his eldest child, the widowed Lady Mary (Michelle Dockery), play the field again. While Tony Gillingham (Tom Cullen) keeps pressing his advantage — even suggesting a shocking romantic getaway at a hotel with doors between their adjoining rooms — Bonneville predicts he will not prevail, if only because Robert simply has terrible judgment.

“Most things Robert says are the opposite of what’s going to take place,” he says, laughing, “so when he remarks, ‘Don’t they look great together?,’ you sort of know that’s not going to happen.”

What’s also not going to happen is that George Clooney, who made news when he appeared in a sketch with the cast for a charity event to be broadcast on ITV, will not be popping in for dinner one night at the estate.

“I have to say that appearance was slightly mishandled by ITV’s press department,” Bonneville says. “It got leaked and journalists put two and two together and made five, of course. Because he was getting married this summer, the rumor was that he had been to Highclere Castle as a wedding venue. And people were saying that he’s going to get married at Highclere Castle. And that was dispelled. And the rumor got started that he was going to be on the show, so therefore he must be playing a wedding guest. He’s not. The point is it’s a standalone sketch for ITV, the broadcaster.”

The producers have hinted that there will be a wedding this season at Downton Abbey, but with Lady Edith in a dither and Lady Mary still thawing out — that, we know, could take another three seasons — we’re likely to see an older couple at the altar.

Might it be Isobel Crawley (Penelope Wilton)?

The prospect of romance returning to her life so unnerves Lady Violet, the Dowager Countess (Maggie Smith), that she actively interferes.

“There’s a lovely thread running through it that is quite poignant with Maggie,” Bonneville says. “Violet realizes that Isobel, much as she spars with her, has actually become quite a pal. So she sort of maneuvers things to try and block the marriage because she doesn’t want to lose her pal.

“I think it’s really charming that you see this late-flowering love for Isobel. And a not dissimilar thing comes over the horizon as well for Violet. As Mary says in one episode, ‘So Granny has a past?’ So that’s quite sweet. There’s something for the grownups, not just the youngsters.”

“Downton Abbey” will go back into production on Season 6 in February. Bonneville says he and the cast spend “an intense six months together” filming. The storyline has already progressed 12 years since the show premiered, but Bonneville says he doesn’t really know Sir Robert Crawley’s age.

“I think we worked it out at one point. It’s his 34th wedding anniversary in Episode 1 and I tried to do the sums on it but I gave up,” he says. “We all have fantastic cosmetics to keep us eternally young.”